Assessment and selection of the recipient cows' corpus luteum at the time of embryo transfer, and its influence on conception rate

Objectives To describe the influence of physical traits of the corpus luteum (CL), as described by transrectal ultrasonography on day 6 post‐oestrus, on the conception rate following embryo transfer (ET) in recipient beef cows. To investigate if higher recipient utilisation rates were achievable, without compromising conception rates to ET. Design/Results Data were analysed from Holstein Friesian embryos (n = 1075) frozen in ethylene glycol thawed for direct transfer into one herd of Angus recipient cows. For pregnancies achieved in the program (n = 693), no statistically significant effect was found for the physical traits of the recipients' CL on conception rate (CL volume ( P  = 0.20), CL side ( P  = 0.14). Conception rates were similar for recipients with a central lacuna (62%, n = 245) and recipients with no central lacuna (66%, n = 448) ( P = 0.10) . Of the pregnant recipients with a central lacuna (n = 245), 98.3% had no remaining luteal cavity by the 30‐day pregnancy ultrasound. No effect on conception rate was found with either the small (<50% of CL diameter) or large (>50% of CL diameter) central lacunae ( P  = 0.18). For recipients with CLs that did not meet previous industry selection guidelines (n = 172, 16% of study population), the conception rate (63%) was not significantly different to the routinely selected recipient CLs (n = 903, conception rate 65%) ( P  = 0.83). Conclusions The suitability of a potential ET recipient is determined by observing an appropriately timed oestrus and a detectable CL, regardless of size or quality.